A transmission electron microscope (TEM) creates images by generating an electron beam that penetrates a very thin specimen. The projected image of electron intensity corresponds to the specimen structure. In order to reduce that intensity image to visible form, the image must be converted to a signal. In a TEM, that signal is generated by impact of the electron pattern onto a scintillator, a photographic emulsion or solid-state imaging plate, or onto the photosensitive element of an electronic camera. A scintillator is normally used for direct, live human viewing or as the input to an electronic camera, which can provide both real-time and recorded images. Photo-emulsions and imaging plates produce recorded images only.
FIG. 1 shows a conventional TEM 10 supported on a table T. TEM 10 includes a housing 12 having an upper cylindrical column or section 12a, which extends down to an enlarged lower section 12b that defines an interior observation chamber 14 at the foot of the column. An operator sitting in front of table T facing housing 12 can view chamber 14 through an x-ray shielding observation window 16 in housing section 12b. Housing 12 is maintained under a relatively high vacuum and an electron gun 18 at the top of section 12a generates an electron beam e within the housing.
Under the influence of electromagnetic lenses (not shown) in column 12a, beam e is transmitted along an optical axis A through a specimen S removably positioned by a support 20 in column 12a. The beam e, now including an electron pattern corresponding to the structure of a selected area of specimen S, is projected onto a scintillator 22a on the upper surface of an opaque direct viewing plate 22 centered on axis A within chamber 14. Invariably the usual plate comprises a relatively thick flat metal substrate whose upper surface is coated with a scintillator such as phosphorus. Typically, plate 22 has a surface area that is large enough to be watched comfortably by an operator looking at the plate through window 16. Such direct viewing plates typically range from 25 to 150 mm in diameter.
In response to the impinging electrons, plate 22 produces a visible image at its upper surface corresponding to the electron pattern which image is visible to the operator looking through window 16. Typically, he/she views plate 22 at an angle of about 45°.
Usually control units 24 and 26 are positioned on table T on opposite sides of housing 12. These units control the TEM, enabling the operator to shift the electron beam e relative to specimen S (or vice versa), so that various areas of the specimen may be viewed on plate 22. Thus, while directly viewing the image on plate 22 and manipulating various controls C on units 24 and 26, respectively, using both hands the operator may align the electron beam and survey the specimen S in a very ergonomic and efficient manner.
Aside from ease of use, many TEM users prefer to operate the TEM while directly viewing the specimen image on plate 22 because TEMs have historically been optimized for this type of observation so that the visible image on the plate 22 is characterized by relatively high resolution, wide dynamic range of response and low apparent noise.
Many TEMs, particularly those built before 2007, usually also include a permanent image recording unit 32 below the observation chamber 14. Since the electron beam e is projected into the recording unit 32, that unit must be maintained under the same vacuum as housing 12 when the TEM is in operation. The unit 32 may temporarily position a recording medium such as film F from the unit's supply tray 32a to a fixed location on axis A as shown before transferring that film to a storage tray 32b. When the specimen S has been surveyed and the desired area thereof has been imaged on plate 22, the plate may be swung up out of the way to the position shown at 22′ so that the electron beam e is projected onto and exposes the film F on axis A, thereby providing a permanent copy of that area of the specimen.
Of course, the recording unit 32 may record images on other recording media such as conventional imaging plates instead of on film F.
As alluded to above, some conventional TEMs also include means for producing real-time images corresponding to the specimen structure captured in the electron pattern in beam e. These images may be displayed on a monitor/recorder 28, positioned on table T next to housing 12. The input signal to monitor 28 may be provided by a conventional electronic camera having a dedicated photosensitive screen which should be perpendicular to the optical axis of the camera to avoid distortion and to maintain high optical performance. This requirement for perpendicularity arises because a high numerical aperture is needed to provide the required sensitivity and resolution for electronic recording. The usual optical couplers and lenses in the camera have a depth of field in the order of only 20 μm so that a deviation from such perpendicularity of only a few degrees would be detrimental to the image focus. This is the main reason why an electronic camera cannot simply acquire pictures suitable for recording on the opaque viewing plate 22 through the observation window 16; i.e. as noted above, that window is oriented at a large angle (45°) with respect to the plate.
In practice, then, the electronic camera that provides the signal to monitor 28 is often mounted in the side of housing column 12a, above chamber 14 as shown at 34 in FIG. 1. The housings of many TEMs include a port 36 with a window 36a in the wall of column 12a for this purpose. In this event, the camera's photosensitive screen 38 may be at the upper surface of a prism 42 located directly opposite port 36 so that screen 38 is perpendicular to axis A. Prism 42 may be mounted to the armature 44a of a linear actuator 44 secured to the side of housing section 12a opposite the camera 34. Under the control of a controller 46, the actuator 44 may move the prism 42 to an extended position shown in solid lines in FIG. 1 wherein the screen 38 is centered on axis A and any visible image thereon is reflected by prism 42 to camera 34 such that the reflected image is also perpendicular to the optical axis of the camera. In response to the incoming image from screen 38, camera 34 provides a signal to monitor 28, causing the monitor to display and/or record that image.
The prism 42 may also be moved by actuator 44 to a retracted position shown in phantom in FIG. 1. In this position, the phosphor screen 38 is not impinged by beam e. Rather, the beam carries on to form a visible image on the direct viewing plate 22 in observation chamber 14. Obviously, the two members 22 and 38 cannot be imaged by beam e simultaneously.
Instead of, or in addition to mounting the electronic camera above the direct viewing plate 22 as described above, some conventional TEMs provide for such mounting on axis A below that plate. In the event that the TEM includes a permanent imagine recording unit such as unit 32 in FIG. 1, the camera may be mounted to the underside of unit 32 as shown at 52 in FIG. 1. For this purpose, the bottom wall of unit 32 may include a port 54 covered by a photosensitive screen 58 optically coupled to the camera by a lens device 59. In such a TEM, means are provided for swinging the direct viewing plate 22 up and away from axis A as shown in phantom at 22′ in FIG. 1. When plate 22 is in this out-of-the-way position and no film F is positioned by unit 32 on axis A, the electron beam e may be projected directly onto the screen 58. The screen thereupon produces a visible image corresponding to the electron beam image of specimen S which visible image is viewed by camera 52 and displayed on monitor 28. Here again, the opaque direct viewing plate 22 and the camera screen 58 cannot be imaged at the same time by the electron beam e.
Conventional electron microscopes such as those outlined above are described, for example, in U.S. Pat. Nos. 4,206,349; 4,739,399 and 5,013,915. Such TEMs are disadvantaged in several respects. First, as noted above, each electronic camera requires its own dedicated photosensitive screen which, when operational, must be located within the evacuated housing 12 on axis A. This requires that the camera and its screen be spaced well above or below the direct viewing plate 22 so as not to interfere with the exposure of the direct viewing plate by electron beam e. When the camera screen is located above the direct viewing plate 22 as at 38 in FIG. 1, that screen must be movable or retractable from axis A so as not to interfere with the imaging of the direct viewing plate 22. This necessitates the presence of the described mechanisms and x-ray-compatible shielding window which increase the overall cost and complexity of the microscope.
On the other hand, if the photosensitive screen for the electronic camera is located below the direct viewing plate 22 as at 58 in FIG. 1, a mechanism must be provided in order to move that plate 22 away from axis A so that beam e can be projected onto the camera screen 58, also adding to the cost and complexity of the instrument. This is especially true of TEMs which include a permanent recording unit such as unit 32 in FIG. 1. Because of the presence of the camera's screen 58, each time a film F is retrieved from unit 32, the entire housing 12 including unit 32 must be pumped down to the high vacuum necessary to subsequently operate the TEM. This also adds to the overall cost of the apparatus.
Finally, it should be emphasized that the electromagnetic optics in a conventional TEM are designed specifically to optimize the image on the direct viewing plate 22 because this is the image that many operators prefer to view through window 16 while aligning the electron beam and surveying specimen S, even though such TEMs may include an electronic camera and associated display monitor. This is because, as noted above and as depicted in FIG. 2, when the camera's photosensitive screen is located above plate 22 as at 38 in FIG. 2, the visible image on that screen 38 produced by beam e is characterized by a lower resolution, e.g. 1-4 megapixels, and higher distortion as compared with the visible image produced on plate 22 which may have a resolution as high as 10-16 megapixels. On the other hand, when the electronic camera's scintillation screen is located below the direct viewing plate 22 as at 58 in FIG. 2, it is highly cropped as compared to the visible image on plate 22. No wonder, then, that many operators prefer to directly view the image on plate 22 when surveying a specimen.